Jump to content


Photo

1/10 wave?

  • Please log in to reply
81 replies to this topic

#1 Jim_Smith

Jim_Smith

    Vostok 1

  • *****
  • Posts: 126
  • Joined: 16 Nov 2012

Posted 22 November 2012 - 11:24 AM

Is there any visual difference at the eyepiece between 1/4.....1/9...1/10 wave mirrors? Thanx Jim

#2 Mirzam

Mirzam

    Skylab

  • *****
  • Posts: 4434
  • Joined: 01 Apr 2008
  • Loc: Lovettsville, VA

Posted 22 November 2012 - 11:41 AM

You may want to peruse this:

http://stellafane.or...ks.html#Quality

JimC

#3 Mike B

Mike B

    Starstruck

  • *****
  • Posts: 9974
  • Joined: 06 Apr 2005
  • Loc: shake, rattle, & roll, CA

Posted 22 November 2012 - 11:48 AM

Here's some more to read!
:grin:

The short answer is, apparently, *yes*, there can be, if all the other aspects that affect optics are properly handled, and the seeing cooperates reasonably well.

#4 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 7594
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 22 November 2012 - 11:54 AM

watch this, it's easy to understand, and its entertaining so get some popcorn. :grin:

Validating Your Mirror Quality

#5 Mirzam

Mirzam

    Skylab

  • *****
  • Posts: 4434
  • Joined: 01 Apr 2008
  • Loc: Lovettsville, VA

Posted 22 November 2012 - 12:23 PM

I was looking for that! Thanks.

JimC

#6 dpwoos

dpwoos

    Apollo

  • *****
  • Posts: 1363
  • Joined: 18 Oct 2006
  • Loc: United States

Posted 22 November 2012 - 01:59 PM

In my experience, the difference is huge. Folks will toss around all kinds of numbers, but the bottom line for me is what I see and the difference between a "diffraction limited" optic and a significantly better one is striking. In fact, I believe that a lot of eyepiece mythology stems from not recognizing this, i.e. that sooper-dooper eyepieces can close the gap, because the mirror can't be the problem. Well, they can't, as a mediocre (diffraction limited) mirror will never produce top-notch views regardless the eyepiece.

I think that some caveats are in order. Any mirror will only perform as good as the collimation, thermal control and seeing allow, and as everyone agrees. However, a factor that maybe is less recognized is the variability in folk's visual acuity. It is not unusual that, at our local club events, not everyone can see whatever we are looking at, and sometimes only a few can. For some of these folks, it may be that the difference between a mediocre and a great optic will go unnoticed. In fact, a friend of mine who was in attendance at the famous Stellafane mirror shoot-out has said that it was surprising to him how many folks claimed to be unable to detect even the most gross differences. In any case, it can be pretty awkward when some folks claim to see nothing while others are all oohing and aahing!

#7 ausastronomer

ausastronomer

    Surveyor 1

  • *****
  • Moderators
  • Posts: 1824
  • Joined: 30 Jun 2003
  • Loc: Kiama NSW (Australia)

Posted 22 November 2012 - 09:05 PM

In my experience, the difference is huge.


I can't quite agree with that comment entirely.

With smaller aperture telescopes (under say 12") the differences are fairly noticeable to a skilled observer under good observing conditions between say a diffraction limited mirror (1/4 wave) and a 1/10th wave mirror. However without star testing under excellent conditions it gets quite difficult to tell the difference between say a 1/7th or 1/8th wave mirror and a mirror which is 1/10th wave or better, for the simple reason a mirror that is a genuine 1/7th or 1/8th wave mirror is an exceedingly good mirror in any case. Any differences between mirrors at this level are very subtle at best and only detectable under the very best conditions.

With larger aperture telescopes (over say 15" aperture) it gets progressively more difficult to separate them, once they are better than diffraction limited, smooth and free of astigmatism. It is certainly still easy to pick a mirror which is worse than diffraction limited (1/4 wave). Larger aperture scopes are more noticeably affected by the subtleties of seeing and thermal equilibrium than smaller scopes. More often than not with a scope of this aperture you are limited by seeing and thermal equilibrium not optical quality, provided the mirror is better than diffraction limited, smooth and free of astigmatism.

What you will invariably find with these larger mirrors, from whichever premium maker you care to choose, is that most of them are no better than 1/6th or 1/7th wave across the full face of the mirror at the worst point. This of course still correlates to a true strehl in the very high 90's and will deliver excellent views.

Cheers,

#8 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 8089
  • Joined: 08 Aug 2007
  • Loc: La Union, PI

Posted 23 November 2012 - 04:33 AM

When the conditions are nearly perfect, the complete package including cooling to ambient, calm seeing, spot on collimation, and well corrected optics combine to form an image that is astounding. Great optics give the focused image that little, but noticeable nudge over the top. Combine that with some observing experience and you truly have a jaw dropping moment. It doesn't get any better than that. IMO, premium is worth it for those moments.

#9 Mark Harry

Mark Harry

    Vendor

  • *****
  • Vendors
  • Posts: 6149
  • Joined: 05 Sep 2005
  • Loc: Northeast USA

Posted 23 November 2012 - 08:18 AM

A really good, smooth 1/10th wave mirror with no edge problems, is hard to beat, or fault with general purpose viewing habits. For planetary/lunar, I would recommend 1/8-1/10th as maximum error for the serious observers. IME, 1/8-1/10 with a good smooth surface and a touch of over/undercorrection can reach 50x/inch, with little to no bloat on stars occurring if conditions permit.
******
My 2 cents, with 1/4 wave and some experience under the belt, I think most observers would start to notice it won't be quite good enough- would lack the astounding characteristic that Norme relates above.
M.

#10 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 8089
  • Joined: 08 Aug 2007
  • Loc: La Union, PI

Posted 23 November 2012 - 08:47 AM

Mark, that's a good point. One might think better correction means it can stand a bit more power per inch. I suspect it does, as long as seeing permits and all the other performance variables are controlled. Not sure if that's accurate, though, since the Airy disc remains the same size. But it might due to less light scattered elsewhere. Contrast might hold up under a bit more magnification. But, higher powers are doable in practice and in good seeing.

#11 dpwoos

dpwoos

    Apollo

  • *****
  • Posts: 1363
  • Joined: 18 Oct 2006
  • Loc: United States

Posted 23 November 2012 - 09:22 AM

I haven't spent all that much time with mirrors bigger than 15", so your comments about big mirrors are interesting. Our club has recently purchased an 18" Obsession, and it disappoints me to say that I am disappointed. On the other hand, our club also purchased a 14" dob, and after a club member refigured the (awful!) mirror it now performs as well as any. A real pleasure to observe with.

Is the big 18" incapable of providing high power images that are as good as the 14", regardless of mirror figure? It may be so, but I won't believe it until I have the opportunity to see this for myself. I believe that the Obsession mirror is guaranteed to be "diffraction limited". There are those in our club who say that the 18" dob is a "light bucket", and not a "planetary scope". However, the same thing was said about the 14", and after that mirror was refigured there is no question that it is an excellent "planetary scope". So, for me the jury is still out on this question for bigger (> 14") mirrors, but I have to admit to being skeptical that something magical happens to collapse the perceived difference between a mediocre optic and an excellent optic when the size goes from 14" to 18", and beyond.

#12 tomharri

tomharri

    Messenger

  • *****
  • Posts: 469
  • Joined: 19 Sep 2008
  • Loc: USA

Posted 23 November 2012 - 10:15 AM

I've had 3x 10" newts over the last 30 years, and you can easily see the difference between a Coulter 'diffraction limited', vs. a strehl .92 Hubble, vs. another Hubble at .98 strehl.

Also mirror mounts are critical especially for thin Hubble mirrors. A 3 point won't allow maximum viewing quality, must be a 9 point floater for 10". And for larger than 14-15" mirrors, must use a 18 point mount to support the mirror properly.

#13 BillFerris

BillFerris

    Soyuz

  • *****
  • Posts: 3687
  • Joined: 17 Jul 2004
  • Loc: Flagstaff, Arizona, USA

Posted 23 November 2012 - 11:16 AM

Here's some more to read!
:grin:

The short answer is, apparently, *yes*, there can be, if all the other aspects that affect optics are properly handled, and the seeing cooperates reasonably well.


The bottom line takeaway for me from the Ceravolo, Dickinson & George article is that a diffraction-limited optic (1/4-wave peak-to-valley on the wavefront) will deliver in-focus views that, to the casual observer, are indistinguishable from those delivered by a 1/10-wave optic. In short, Lord Rayleigh's standard remains relevant as a measure of an optical system's performance.

As the authors found, an experienced observer who knows what to look for is able to discern a modest improvement in views delivered by a 1/10-wave system when compared, side-by-side, to a 1/4-wave system under very good to excellent seeing conditions. A 1-wave system should be obviously limited in performance even to the casual observer. A 1/2-wave system should reveal itself as limited after not much time or effort. But 1/4-wave and better systems require time and the right conditions to distinguish.

This is not to say a 1/10-wave system isn't worth the expense. For some observers, just the knowledge that the view delivered by their scope isn't indistinguishable from perfect is enough to limit their enjoyment. But for most observers, a diffraction limited scope is going to deliver crisp, detailed views and years of enjoyable use.

Bill in Flag

#14 dpwoos

dpwoos

    Apollo

  • *****
  • Posts: 1363
  • Joined: 18 Oct 2006
  • Loc: United States

Posted 23 November 2012 - 12:57 PM

For sure that is what they say, and I don't agree at all. These three are very respected, and so it gives me no pleasure to disagree. However, I can only say that I see what I see, and what other folks that I know and respect as much as these three claim to see. I would advise anybody who cares about this to visit their local astro club, where they can use all kinds of scopes with all kinds of optical quality, and judge for themselves.

A club member with a commercial/Chinese 10" dob recently did just that, and he ended up having our club's best mirror maker do a refiguring job. I don't recall specifically looking through his scope before the work, but now it is a real joy to observe with and he seems elated with the improvement. In my experience this story is not uncommon. Anecdotal evidence only? For sure, but so is the evidence to the contrary even though it comes from respected folks.

#15 GeneT

GeneT

    Ely Kid

  • *****
  • Posts: 12650
  • Joined: 07 Nov 2008
  • Loc: South Texas

Posted 23 November 2012 - 01:31 PM

Yes.

#16 ausastronomer

ausastronomer

    Surveyor 1

  • *****
  • Moderators
  • Posts: 1824
  • Joined: 30 Jun 2003
  • Loc: Kiama NSW (Australia)

Posted 23 November 2012 - 05:16 PM

I haven't spent all that much time with mirrors bigger than 15", so your comments about big mirrors are interesting. Our club has recently purchased an 18" Obsession, and it disappoints me to say that I am disappointed. On the other hand, our club also purchased a 14" dob, and after a club member refigured the (awful!) mirror it now performs as well as any. A real pleasure to observe with.

Is the big 18" incapable of providing high power images that are as good as the 14", regardless of mirror figure? It may be so, but I won't believe it until I have the opportunity to see this for myself. I believe that the Obsession mirror is guaranteed to be "diffraction limited". There are those in our club who say that the 18" dob is a "light bucket", and not a "planetary scope". However, the same thing was said about the 14", and after that mirror was refigured there is no question that it is an excellent "planetary scope". So, for me the jury is still out on this question for bigger (> 14") mirrors, but I have to admit to being skeptical that something magical happens to collapse the perceived difference between a mediocre optic and an excellent optic when the size goes from 14" to 18", and beyond.


I have to admit to being skeptical that something magical happens to collapse the perceived difference between a mediocre optic and an excellent optic when the size goes from 14" to 18", and beyond.


This isn't the case at all. What happens is the number of nights where you can actually see that one mirror is better than the other reduces dramatically as the aperture increases. Mirror makers going to thinner 1.6" thick mirrors in larger apertures has helped dramatically in this regard but an 18" mirror under unfavourable thermal conditions can still take plenty of cooling, even if it is thin. Further as the aperture increases it takes progressively better seeing conditions for larger scopes to deliver their best images. For instance I have seen nights where my 14" scope will hold up at 400X but due to a combination of thermal equilibrium issues and variable seeing the 18" scope will not get past 250X. When everything comes together with the 18" scope it is an excellent planetary scope. I have had it to 1075X on the Moon and Saturn on a couple of occasions. It's just that as the aperture increases these occasions become less frequent.

We have 4 18"/F4.5 classic Obsessions in our 3RF arsenal of scopes. One of these has a .90 strehl mirror, the other 3 have mirrors with a strehl > .97. These mirrors have been interferometrically tested. On one occasion only in the 7 years we have had these scopes in our care, have I been able to pick the lower graded mirror apart from the others and that was a feature on Jupiter which was barely detectable; and that could have even been attributed to variable seeing as I changed scopes. That having been said that mirror is still a very good mirror. It's smooth with an excellent edge and slight undercorrection. In terms of peak to valley its worst point on the mirror face is 1/4.5 waves. That mirror is in fact the mirror in my scope and it can still push 1075X under favourable conditions. Would I have gained anything with 1 of the other mirrors, yeah something barely detectable once in 7 years.

Cheers,

#17 rguasto

rguasto

    Viking 1

  • *****
  • Posts: 768
  • Joined: 18 Nov 2010
  • Loc: Long Island, NY

Posted 23 November 2012 - 07:05 PM

yeah something barely detectable once in 7 years.


Well said. Very nice.

Image quality IMO is more dependent on seeing which we have no control over. The best optics and eyepieces still won't help bad seeing conditions. We're at the mercy of the atmosphere. The best views I've ever had were with telescopes of "average" (no apo's, all commercial mirrors) optical quality, 8" aperture and under but the atmosphere was very, very forgiving. 1/4 wave, 1/10 wave?.....................

-Rob

#18 dan_h

dan_h

    Surveyor 1

  • -----
  • Posts: 1966
  • Joined: 10 Dec 2007

Posted 23 November 2012 - 08:47 PM

Image quality IMO is more dependent on seeing which we have no control over. The best optics and eyepieces still won't help bad seeing conditions. We're at the mercy of the atmosphere. The best views I've ever had were with telescopes of "average" (no apo's, all commercial mirrors) optical quality, 8" aperture and under but the atmosphere was very, very forgiving. 1/4 wave, 1/10 wave?.....................

-Rob


But if you were to get out of NY and relocate somewhere that has routinely excellent seeing, your point of view would change. Under premium conditions, premium optics rule.

dan

#19 ausastronomer

ausastronomer

    Surveyor 1

  • *****
  • Moderators
  • Posts: 1824
  • Joined: 30 Jun 2003
  • Loc: Kiama NSW (Australia)

Posted 23 November 2012 - 09:32 PM

I don't live anywhere near New York. In fact I have some premium optics, observe under Bortle I skies day in day out and haven't changed my opinion on this when it comes to larger optics.

Cheers

#20 ed_turco

ed_turco

    Apollo

  • *****
  • Posts: 1384
  • Joined: 29 Aug 2009
  • Loc: Lincoln, RI

Posted 24 November 2012 - 11:49 AM

True, under mediocre seeing, once in a great while, the seeing, by accident, becomes much better. And then you will thank your lucky stars that you have 1/10 wave mirror! Images become close to miraculous!

Ed

#21 rik ter horst

rik ter horst

    Mariner 2

  • -----
  • Posts: 221
  • Joined: 01 Nov 2010
  • Loc: Ewer, the Netherlands

Posted 24 November 2012 - 01:54 PM

We have 4 18"/F4.5 classic Obsessions in our 3RF arsenal of scopes. One of these has a .90 strehl mirror, the other 3 have mirrors with a strehl > .97. These mirrors have been interferometrically tested. On one occasion only in the 7 years we have had these scopes in our care, have I been able to pick the lower graded mirror apart from the others and that was a feature on Jupiter which was barely detectable; and that could have even been attributed to variable seeing as I changed scopes. That having been said that mirror is still a very good mirror. It's smooth with an excellent edge and slight undercorrection. In terms of peak to valley its worst point on the mirror face is 1/4.5 waves. That mirror is in fact the mirror in my scope and it can still push 1075X under favourable conditions. Would I have gained anything with 1 of the other mirrors, yeah something barely detectable once in 7 years.

Cheers,


Interesting, your experience! I believe you should be very happy with your mirror (and you are I understand!). The mirrors have been tested with an interferometer in a laboratory under stable conditions I guess, and your particular mirror is undercorrected. Couldn't be much better because a 100% corrected large mirror tends to become slightly overcorrected during cool down because the edge of the mirror cools faster than the rest. Although your mirror has been measured to be Lambda/4.5 wave P-V undercorrected I expect it to be better than that during observation, at least as long as the temperature drops....

This might explain the almost invisible difference in performance.


Cheers,
Rik

#22 dan_h

dan_h

    Surveyor 1

  • -----
  • Posts: 1966
  • Joined: 10 Dec 2007

Posted 24 November 2012 - 01:54 PM

I don't live anywhere near New York. In fact I have some premium optics, observe under Bortle I skies day in day out and haven't changed my opinion on this when it comes to larger optics.

Cheers


The worst mirror you referenced was a Strehl 0.90 which is already a pretty decent optic. Do you think you would see the difference between it and a 0.8 Strehl? Under excellent skies, I think it would stand out to all but the most casual of observers.

dan

#23 csrlice12

csrlice12

    Voyager 1

  • *****
  • Posts: 10582
  • Joined: 22 May 2012
  • Loc: Denver, CO

Posted 24 November 2012 - 02:30 PM

Regarding 1/10 wave mirrors in a Diagonal for a refractor. Celestron has a 1/10 wave XLT diagonal, any idea how it compares to other diagonals?

#24 Asbytec

Asbytec

    Guy in a furry hat

  • *****
  • Posts: 8089
  • Joined: 08 Aug 2007
  • Loc: La Union, PI

Posted 24 November 2012 - 03:15 PM

With the amount of light cast into the rings with 1/4 wave spherical, one should notice a difference when seeing is calm enough and all other variables controlled. Especially doing a side by side comparison of the focused image. When seeing, cooling, and collimation are combined with better correction, IMO, you will be astounded. The better correction just gives it that nudge over the top during those moments. The difference between Strehl 0.8 and perfect 1 should be apparent while 0.95, give or take, is quite good enough to show improvement.

#25 ausastronomer

ausastronomer

    Surveyor 1

  • *****
  • Moderators
  • Posts: 1824
  • Joined: 30 Jun 2003
  • Loc: Kiama NSW (Australia)

Posted 24 November 2012 - 06:32 PM

The worst mirror you referenced was a Strehl 0.90 which is already a pretty decent optic. Do you think you would see the difference between it and a 0.8 Strehl? Under excellent skies, I think it would stand out to all but the most casual of observers.

dan


Hi Dan,

I discussed this in one of my earlier posts.

With larger aperture telescopes (over say 15" aperture) it gets progressively more difficult to separate them, once they are better than diffraction limited, smooth and free of astigmatism. It is certainly still easy to pick a mirror which is worse than diffraction limited (1/4 wave).


The point I am trying to make is that with larger mirrors it's easy to detect a mirror that is less than good to very good, but you need progressively better seeing and thermal stabilisation to detect any gain once the mirror gets to this level and even then the gain is very subtle.

The difference with smaller mirrors (say under 14" to 15") is a lot more noticeable and on a lot more occasions.

Cheers,






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics